The present invention relates to vapor compression refrigeration systems and more particularly relates to refrigerant expansion devices for use in such systems.
There are many situations in which it is desirable to change the bore (restriction) size of a refrigerant expansion device in response to the temperature of the refrigerant passing through the device. For example, an air conditioner or a heat pump used to cool a house may have a refrigerant expansion device, located inside the house, for controlling refrigerant flow from an outdoor heat exchange unit to an indoor heat exchange unit. If the outdoor ambient temperature is relatively high then there may be some floodback of liquid refrigerant to the compressor because of the relatively small pressure drop across the refrigerant expansion device due to the relatively high temperature and pressure of the liquid refrigerant flowing to the device from the outdoor heat exchange unit. Floodback is prevented if there is a decrease in bore size of the refrigerant expansion device in response to an increase in temperature of the refrigerant flowing through the device. The smaller bore size increases the pressure drop across the device to ensure that all the liquid refrigerant flowing to the indoor heat exchange unit is vaporized.
Also, in a home heat pump system having an outdoor refrigerant expansion device, when the system is operating in the heating mode it is desirable to increase the bore size of the refrigerant expansion device in response to a relatively low temperature refrigerant flowing through the device to maintain proper system operation under conditions such as a large reduction in indoor temperature during a period of thermostat setback. This is true because normally in the heating mode the liquid refrigerant flowing from the indoor heat exchange unit to the outdoor heat exchange unit is at a temperature slightly above the indoor air temperature and this liquid refrigerant will become cooler with decreasing indoor air temperatures experienced during periods of thermostat setback. This decrease in temperature of the liquid refrigerant flowing to the outdoor heat exchange unit may result in undesirable frosting over of the outdoor heat exchange unit and/or an undesirable reduction in vapor flow to the compressor. These undesirable events may be prevented by increasing the bore size of the outdoor refrigerant expansion device, thereby increasing the rate of refrigerant flow to the outdoor heat exchange unit during such periods of reduced condensing temperature and increased subcooling.
Further, in a heat pump system having an indoor expansion device, it is desirable to increase the bore size of the device in response to relatively low refrigerant temperatures during the initial portion of defrost cycles. This is true because upon initiation of a defrost cycle, the heat pump system operates with a very low discharge pressure due to the relatively cold outdoor heat exchange unit which results in relatively cold liquid refrigerant flowing from the outdoor heat exchange unit to the indoor heat exchange unit.
This low discharge pressure results in less than a desirable amount of refrigerant flow through the expansion device. Defrost performance is improved by increasing the bore size of the refrigerant expansion device during the first portion of the defrost cycle and then changing to normal bore size later in the defrost cycle when the outdoor heat exchange unit begins to warm.
There are refrigerant expansion devices which may be suitable for use in the above-described situations. For example, temperature responsive capillary tubing and other such devices made from dissimilar metals having different thermal expansion coefficients may be used to provide an expansion device having a bore size which changes in response to the temperature of the liquid flowing through the device. However, the bore size of these devices does not undergo a single dramatic change at a given temperature bit, instead, undergoes continuous change depending on the temperature of the device. Also, these devices are relatively complex in structure and relatively difficult to manufacture because of the necessity for joining the dissimilar metals to form a bore having temperature sensitive walls made of the dissimilar metals. Special cuts, notches, and other configurations for the metals are usually required to produce special shapes for the bore walls so that the walls are temperature responsive. Also, the dissimilar metals are usually joined by welding, brazing, or soldering thereby requiring special manufacturing steps.